Methods and systems for producing gasabsorption lines of doppler-reduced breadth



Aug. 26, 1958 R. METHODS AND SYSTEMS FOR PRODUCING GAS-ABSORPTIO flff-r 6075 H DICKE LINES 0F DOPPLER-REDUCED BREADTH Filed Jan. 31, 1956 aan sie.'

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INVENTOR. HUBERT H. DI :Ki:-

METHDS AND SYSTEMS FR PRODUCING GAS- ABSORPTION LINES F DOPPLER-REDUCED BREADTI-I Robert H. Dicke, Princeton, N. J., assigner, by mesne assignments, to the United States of America as represented by the Secretary of the Army Application Ilanuary 31, 1956, Serial No. 562,441

7 Claims. (Cl. Z50-36) This invention relates to methods and systems involving the utilization of molecular resonances of gases at microwave frequencies.

In accordance with the present invention, a microwave carrier is pulse-modulated to provide microwave energy having a plurality of sideband frequencies one of which corresponds with or may correspond with a molecular resonance of a gas. The pulse-modulated energy is propagated through the gas at low pressure along a path in which the excited gas molecules are subjected to a spatially periodic field. ln consequence, there is strong sharp absorption upon coincidence between a molecular resonance and one of the sidebands providing a microwave signal of rigidly predetermined frequency which is substantially free of the Doppler-broadening otherwise arising because of random motion of the excited gas molecules.

Further in accordance with the present invention, the unmodulated microwave carrier and the microwave signal from the gas are mixed to produce a distinctive diierence-frequency signal useful in microwave spectroscopy to analyze or monitor the composition of a gas, and useful with a known gas to effect rigid stabilization of a microwave source at a predetermined desired operating frequency.

More particularly, and for frequency-control,purposes, the difference-frequency signal and a reference signal derived from the modulating source for the microwave carrier are impressed upon a detector to produce a control signal of reversible polarity corresponding with the deviation from normal of the carrier frequency and of rapidly changing amplitude for small deviations.

The invention further resides in molecular resonance systems having features of novelty and utility hereinafter described and claimed.

For a more detailed understanding of the invention and of systems embodying it, reference is made in the following description to the accompanying drawings in which:

Fig. l is a schematic circuit diagram, in block form, of a frequency-stabilizing system using a Doppler-reduced absorption line as a frequency standard;

Fig. 2 schematically illustrates the gas cell shown in Figs. l, and 6;

Figs. 3 and 4 are explanatory figures referred to in discussion of Figs. 1, 5 and 6; and

Figs. 5 and 6 are modifications of the system shown in Fig. l.

Referring to Fig. l, the source of microwave oscillations may be a magnetron, a klystron or other microwave generator provided or associated with resonant cavity structure serving as a ilywheel primarily determining the operating frequency and affording a stiff control subject however to drift such as due for example to changes in ambient temperature.v

As now well known, many gases exhibit, at suitably low pressure, molecular resonance frequencies which are characteristic of a particular gas and which are not United States Patent affected by temperature or other usual ambient conditions. One of such gases, for example ammonia, is confined cell l1 at a pressure of about 104 mm. of mercury. The cell is coupled as by waveguide transmission line 12 to the microwave source 10.

During its transmission to cell 11, the microwave energy is pulse-modulated to produce a multiplicity of sidebands, one of which is made to correspond in frequency with a molecular resonance frequency of the gas in cell 11. So to modulate the microwave carrier, the transmission line 12 may include a hybrid junction 13, such as a magic-tee one of whose branches (14) is provided with a modulator 1S, such as a crystal diode, coupled by a probe to the microwave field. The path of the microwave carrier to modulator 15 is indicated by the upper unfeathered arrow.

The modulator 15 is also connected through an electronic gate 30 to a source 16 of relatively low-frequency oscillations. Pulses from a suitable pulse-generator 7 periodically open the gate 30 to provide a path for transmission of the lower frequency oscillations from source 16 to modulator 15. The periodic pulsing of the lowfrequency oscillations produces a group of frequencies, all of which through action of modulator 15 interact with the microwave carrier from source it) to produce a group of microwave frequencies.

This group of frequencies, as indicated by the upper feathered arrow, is transmitted by branch or arm 17 of the hybrid junction-an effective continuation of transmission line 12-10 the gas cell 11 which is of the parallei-grid type shown in one or the other of my prior applications Serial Nos. 243,082 or 430,855 respectively filed on August 22, 1951 and May 19, 1954.

Specifically referring to Fig. 2 hereof, a series of parallel grid structures is disposed in pairs 26, 27 within cell ilA with the planes of the grids normal to the direction of propagation iu the cell of the pulsed microwave energy supplied from the hybrid junction. The grid structures are permeable to the gas and to the propagated microwave energy. The windows 9, 9 are permeable to the microwave energy but confine the gas within the cell.

The grids 26, 2'7 of each pair are at the same fixed direct-current potential whereas the successive pairs of grids are at successively higher or successively lower potentials, differing by a iixed amount E. Specifically, the successive pairs of grids are connected to points along a potential-divider 28 supplied from a direct-current source exemplified by battery 29. The resitors comprising the potential-divider are of equal value, affording the aforesaid frxed difference E between the potentials of the succcssive pairs of grids.

From the foregoing it will be understood that a fieldfree region exists between the grids of each pair and that constant Stark field regions of equal intensities exist between successive pairs of grids. Assuming that the gas is ammonia, the perturbed line frequency (fgs) in these constant fields is where fg is the unperturbed line frequency B is a constant.

Because ofl movement of any individual gas molecule, absorption by it occurs at the frequency where v9 is the mean absorption frequency of a stationary v is the component of molecule velocity in the direction of propagation of microwave energy in the gas cell and c is the velocity of light. Because such motion is through the spatially periodic eld produced by the grid wires or equavalent, the absorption is frequency-modulated with a fundamental frequency of With a grid spacing of or more generally of (where k is the wavelength of the microwave energy as propagated in the gas cell and n is a small integer), ab-

sorption occurs at:

v v v 1 :l: m210- (where m is an integer).

Because of the spatially-periodic field, all velocity classes of the gas molecules absorb microwave energy incident on the cell 11A in the intervals between pulses to afford the sharp absorption spike S of Fig. 3.

In the particular gas cell shown in Fig. 2, the microwave energy including the carrier and the sidebands after once transversing the spatially periodic field is reflected back by the tuning plunger 32 so that it again traverses the eld for enhanced Doppler reduction of the bandwidth of the absorption. All of these frequencies are propagated in branch 18 of the hybrid junction (Fig. 1) and impressed upon mixer 19 which may be a crystal diode. As indicated by the lower unfeathered arrow, the unmodulated microwave carrier is also impressed upon mixer 19 to produce a beat-frequency signal. Because of the propogation characteristics of the hybrid junction, whether it be a magic-tee or an arrangement of directional couplers, the sideband energy is not to significant extent impressed upon mixer 19 until after it has passed through the cell 11.

When a sideband frequency coincides with a molecular resonance frequency of the gas, the energy of that sideband is sharply selectively absorbed as indicated in Fig. 3. This sideband excites the gas and there persists in the interval between successive pulses a microwave signal at the same frequency as the sideband energy which can be -considered as a microwave signal radiated by the gas. This radiation signal interferes destructively with the original exciting sideband and provides a steady microwave signal continuously beating with the unmodulated carrier to produce a mixer output signal of beat-frequency fb.

During the relatively short duration of each pulse, the mixer 19 is saturated by the strong pulsed signal from modulator 15 and has negligible beat-frequency output. The beat-frequency signal due to radiation from the gas persists in the relatively long interval between the successive exciting pulses and provides a strong sharp frequency standard Well suited for rigid stabilization of the microwave source at a desired operating frequency.

By way of specific example, it is assumed that the desired normal operating frequency (fo) of source 10 is 23,840 megacycles and that the selected frequency standard is the 3, 3 line of ammonia which exists at the molecular resonance frequency (fg) of 23,870. In such case, the xed frequency (fm) of source 16 may be selected as 30 megacycles and the parameters of pulser 7 may be chosen to generate one-microsecond pulses at a repetition rate of 100,000 per second. The output from gate 30 is a group of frequencies including in addition to the frequency of 30 megacycles an ascending series of frequencies 30.1, 30.2, 30.3 megacycles and a descending series of frequencies 29.9, 29.8, 29.7 megacycles. This group of frequencies is dependent -on the pulse repetition rate and is determinable by Fourier analysis, and may be more generally dened as (Z) (mipwmcwhere p is' a pulse repetition rate and n is a harmonic number=0, 1, 2, 3

This group of frequencies, as applied to modulator 1S modulates the microwave carrier to produce a multiplicity of microwave frequencies which in the particular example above are of the frequencies (3) i01- [30i0.ln] megacycles.

All of these frequencies, as indicated by the upper feathered arrow, are transmitted by arm 17 of the hybrid junction to gas cell 11. As indicated in Fig. 1 by the lower feathered arrow, all of these frequencies after traversing cell 11 in one direction are reflected back through arm 17 of the hybrid junction, are propagated in branch 18 thereof, and are there impressed upon mixer 19. As indicated by the lower unfeathered arrow, the unmodulated microwave carrier is also impressed upon mixer 19.

From Equation 3 above, it will be noted that one of the microwave sidebands (23,870 mc.) coincides with the selected molecular resonance of the gas in cell 11 when the operating frequency of source 10 is 23,840 mc. Hence this sideband energy excites the gas as above described to produce a strong sharp signal (Spike S of Fig. 3) which beating with the carrier in mixer 19 produces an intermediate frequency signal sharply peaked at 30 mc.

The beat-frequency signal, preferably after amplification to suitably high level by frequency-selective amplifier 20 is impressed upon one input circuit of detector 21 which may be any of the known phase-sensitive types. Upon the other input circuit of detector 21 is impressed a reference signal derived from the xed low-frequency source 16. The output signal of detector 21 therefore varies as a function of the operating frequency of microwave source 10.

To obtain from detector 21 an output which contains information concerning both the sense and extent of the deviation from the desired operating frequency of microwave source 10, phase-shifter 22 is interposed between the source 16 and detector 21. The four curves shown in Fig. 4 are for four settings of the phase-shifter which respectively correspond with successive shifts in the same direction. Each curve A-D shows the variation of the output voltage of detector 21 as a function of the frequency of microwave source 10 for a corresponding one of the four aforesaid settings of the phase-shifter 22.

Curves B and D are absorption-response curves of the gas, each having a maximum when the operating frcquency fo of the microwave source is at the desired value: however, all other values of these curves are ambiguous in that they do not convey the sense of the deviation. Curves A and C which are dispersion-response curves of the gas, each has a zero value for null deviation of the operating frequency fu from the desired value and each has rapidly increasing values in opposite directions or polarities for increasing deviation in opposite directions of the operating frequency fo.

With the phase-shifter 22 set for either of curves A or C, the detector output is a direct-current signal which throughout the range of -Afo to -i-Ao corresponds in polarity and amplitude with the deviation from the desired frequency of operation of microwave source 10. For either of these' two corresponding phase-shifter settings, an` appropriately marked direct-current meter 23 may be used to indicate to an operator the sense and extent to which the `frequency of source 1t) should be varied, as by re-tuning of the frequency-control cavity or adjustment of the potential of a control electrode of the oscillator tube or an associated reactance tube, to return the operating frequency fu to normal. Which of the two curves should be used in automatic stabilization of a given installation depends upon the characteristics of the particular system used to vary the frequency of source 10. Various systems for using a direct-current Voltage of reversible polarity to vary the frequency of an oscillator are known and n eed not be here described.

For the curves A and C, the rate of change with frequency of the D. C. control voltage is high within the narrow range defined by limits -Af0, -l-AD of Fig. 4. In the particular case under discussion, this range is only about tive to seven kilocycles at a normal operating frequency of 23,840 mc. for microwave source 10. This is exemplary of the rigidity of the frequency-stabilization realizable by pulse-modulation of the microwave energy supplied to a gas cell having a spatially periodic eld.

Preferably the output circuit of the phase-sensitive detector 21 includes a low-pass filter 31 to attenuate yany alternating-current components of the detector output which might otherwise aifect the deviation meter 23 or the final control element of an automatic frequency-control system.

In the system of Fig. 1, the microwave source need not be a self-excited oscillator source of sustained microwave oscillations. As schematically indicated in Fig. 5, the source may be a microwave power amplifier excited from the last stage of a series of frequency-multipliers or of `a harmonic amplier chain whose first stage is driven from a relatively low-frequency oscillator stiifened by a high Q frequency-determining circuit or element such as a piezo electric crystal. By way of example, the oscillator 25 may be' stabilized by a quartz crystal at .a normal operating frequency of 23.84 mc. and the harmonic amplifier chain 24 may provide a multiplying factor of 1000 so that the microwave oscillations from source 10 are normally at the frequency of 23,840 megacycles as in the above specifically discused example of Fig. 1. In this case, the direct-current signal output of detector 21 may be used to stabilize the frequency of oscillator 25 in any manner known per se as by varying the bias of a reactance tube or by varying the temperature setting of an oven for the quartz crystal.

In other respects of composition and operation, the system of Fig. 5 is similar to that of Fig. 1 and need not be further described.

In the system shown in Fig. 6, the tuning plunger or other structure for reflecting the microwave energy back through the gas cell is omitted and a different hybrid junction arrangement is used to prevent impression upon the mixer 19 of the microwave sidebands before their transmission through the gas cell 11B. Specifically, the unmodulated microwave carrier from source 10 is continuously impressed upon modulator 15 through a path including directional coupler 14A and upon the mixer 19 through `a path including the directional coupler 18A. These paths are indicated by the unfeathered arrows.

The intermittent or pulsed microwave sidebands, as indicated by the feathered arrows, are propagated in a path including the directional coupler 14A, gas cell 11B and directional coupler 18B. In the interval between pulses, the microwave signal radiated by the gas and containing the phase information concerning deviation from the desired operating frequency of microwave source 10 is transmitted to the mixer 19 in a path including the directional coupler 18B. It is there mixed as above described with the unmodulated carrier to provide the lowfrequency signal which is beat against a reference signal from source 16 to provide in the output of detector 21 a D. C. control signal of polarity and amplitude corresponding with the deviation of the operating frequency of microwave source 10 from the desired value.

In the -systems of Figs. 1, 5, and 6 as above described, the standard-frequency signal is used to stabilize the frequency of source 10. The high resolution afforded by applying pulsed microwave energy to a gas cell of the spatially-periodic iield type may also be utilized to advantage in microwave spectroscopy for analysis of gas mixtures, particularly those having components exhibiting molecular' resonances at closely adjacent frequencies. In such case, the intermediate frequency (fm) of oscillator 16 may be adjusted to different values each affording a different group of sideband frequencies which interlace the frequencies of the group fixed by another intermediate frequency value. Thus by checking the frequencies at which source 10 of the spectrograph is stabilized for different values of frequency fm, the identity of a particular gas or gases exhibiting molecular resonances in the investigated range may be clearly established. Such checking may be done by measuring the difference in frequency between the microwave source 10 of the spectrograph and the known frequency of a microwave source o-f a second system stabilized as in Figs. 1, 5 or 6.

A typical set of operating conditions under which the presence of ammonia may be established are given above. By shifting the frequency of source 16 to 14.2 mc., the sideband frequencies 23,854.2 mc. and 23,825.8 mc. are generated `assuming the frequency of source 10 remains at 23,840 mc. One of these (23,854.2 mc.) corresponds with a line of methyl alcohol and hence presence of that gas produces a strong absorption for this different value of frequency fm of the intermediate-frequency source. This line for methyl alcohol (CI-14D or CHSOH) is given on page 99 of the National Bureau of Standards Circular 518.

In like manner any other portion of the microwave spectrum may be investigated to ascertain the identity of the gas or gases of a given sample. In a given industrial process, it may be necessary only to detect the presence of one contaminant so that it sufces to leave the frequencies fo and fm set for the values for which one of the microwave sidebands is known to coincide with a molecular resonant frequency of that contaminating gas. The presence of the contaminant gas can be detected adjusting the phase shifter 22 and observing the meter 23.

Arrangements such as shown in Figs. l, 5 and 6 may be used to advantage in microwave spectroscopy without using the D. C. output of detector 21 to minimize frequency-drift of source 10. With the frequencies fo and fm set to different known values, the rnicro-wave sideband frequencies are known and if for any of such settings there occurs a sharp absorption, the identity of a component of the gas can be established from comparison of the observed absorption frequency with the known absorption frequencies of various gases for that portion of the spectrum.

In all of the foregoing examples in frequency-stabilization and microwave spectroscopy, the significant advance is the Doppler-reduction in width of the absorption line obtained by impressing pulse-modulated microwave energy upon a gas cell having a spatially-periodic field which remains constant with time.

What is claimed is:

1. A molecular resonance system comprising a gas cell for confining at low pressure a gas which exhibits molecular resonance and for providing a propagation path for microwave energy, a plurality of conductive structures permeable to the gas and to said microwave energy disposed within said cell and spaced in the direction of propagation of said energy, means for generating microwave energy, means for pulse-modulating said microwave energy to produce microwave sidebands,

means for introducing said pulse modulated energy into said gas cell, and means for exciting said structures to produce within said cell a spatially periodic field` for Doppler-reduction in line breadth of the absorption of one of said microwave sidebands by the gas.

2. A molecular resonance system comprising a gas cell for confining at low pressure a gas which exhibits molecular resonance and for providing a propagation path for microwave energy, a plurality of conductive structures permeable to the gas and to said microwave energy disposed within said cell and spaced an integral number of quarter-wavelengths of said energy in, the direction of propagation thereof, means for generating microwave energy, means for pulse-modulating said microwave energy to produce microwave sidebands, means for introducing said pulse modulated energy into said gas cell, and means for exciting said structures to produce within said cell a spatially periodic field for Doppler-reduction in line breadth of the absorption of one of said microwave sidebands by the gas.

3. A molecular resonance system comprising a gas cell for confining at low pressure a gas which exhibits molecular resonance absorption and for providing a propagation path for microwave energy, a plurality of conductive structures disposed within said cell and spaced an integral number of quarter-wavelengths of said energy in the direction of propagation thereof, a source of low frequency, means for pulse modulating said low-frequency source to produce low-frequency sidebands, means for generating microwave energy, means for utilizing said low frequency sidebands to modulate said microwave energy to produce microwave sidebands, one of which is selectively absorbed by the gas, means for introducing said modulated microwave energy into said gas cell, and means for exciting said conductive structures to produce within said cell a spatially periodic eld effecting Doppler-reduction in bandwidth of said selective absorption by the gas of said one of the sidebands.

4. A molecular resonance system as in claim 3 additionally including a mixer upon which is impressed the unmodulated microwave energy and the microwave energy appearing at the output of the gas cell to produce a strong sharp difference-frequency signal.

5. A system as in claim 3 in which the gas cell is a high Q resonant chamber tuned for shock-excitation by said one of the sidebands for persistence of radiation by s the gas at a frequency correspondingv to' the sideba'nd frequency during the intervals between successive pulses.

6. A systemy for stabilization of the frequency of microwave carrier energy comprising a cell containing gas at low pressure for which it exhibits molecular reso'- nance absorption and provides a propagation path' for microwave energy, a plurality of conductive structures permeable to the gas and to said microwave energy disposed within said cell and spaced any integral number of quarter-wavelengths in the direction of propagation of said energy, a source of low frequency, means for pulse modulating said low-frequency source to produce low frequency sidebands, means for generating microwave energy, means for utilizing said low frequency sidebands to modulate said microwave energy to produce microwave sidebands, one of which normally corresponds with a molecular resonance frequency of the gas, means for introducing said modulated microwave energy into said gas cell, means for exciting said conductive structures to produce within said cell a spatially periodic field effecting Doppler-reductionl in bandwidth of said selective absorption by the gas of one of the sidebands, mixer meansy coupled to the output of said gas cell and to said microwave generator for beating the unmodulated microwave carrier against the microwave energy appearing at the output of said cell, a phase-sensitive detector having one of its input circuits coupled to the output of said mixer means, and a phase-shifter coupled between said lowfrequency source andl the remaining input circuit of said detector whereby thev output ofy saidy detector is of polarity dependent upon the sense of deviation from the normal frequency of the microwave carrier and of rapidly changing amplitude for small magnitudes of such deviation.

7. A system as inclaim 6 in which amplifier means is interposed between said mixer and saiddetector to se'- lectively amplify said beat-frequency to the substantial exclusion of other beat-frequencies between said carrier energy and the other of said sidebands.

NortonI Feb. 5, 1952 Norton v May 8, 1956 

